1. Field of the Invention
The present invention relates to an improvement in a so-called "controlled type rotation speed difference sensitive coupling" which is used in, for example, a driving power distributing device for a four-wheel drive motor vehicle, a differential gear arranged between left and right drive road wheels, a differential limiting device arranged between front and rear wheels and/or left and right drive road wheels, and the like.
2. Description of the Prior Art
In order to clarify the task of the present invention, one controlled type rotation speed difference sensitive coupling will be outlined with reference to FIGS. 23 and 24 of the accompanying drawings, which is disclosed in U.S. patent application Ser. No. 07/409,669 now U.S. Pat. No. 5,024,309 which was filed Sep. 20, 1989 in the name of Toji TAKEMURA et al.
The following description on the previously proposed coupling may be easily understood when taken in conjunction with FIGS. 5 and 6 because these drawings show a similar construction to the previously proposed coupling.
As may be seen from these drawings, the coupling comprises generally a drive housing (30), a rotor (40) rotatably disposed in the drive housing (30), six driving pistons (50) radially movably received in evenly spaced cylinder bores (42) formed in the peripheral portion of the rotor (40), six working chambers (60) defined in the cylinder bores (42) behind the pistons (50), six balance fluid passages (70) formed in the rotor 40 to communicate the working chambers (60) with a spool chamber (90) formed in the center of the rotor (40), six regulator relief fluid passages (80) formed in the rotor 40 to communicate the working chambers (60) and the spool chamber (90) under a given condition, a spool (73) movably disposed in the spool chamber (90) and an external actuator (not shown) which actuates or moves the spool (73) through a rod (74) extending from the spool (73).
Each of the balance fluid passages (70) has, at a portion exposed to the spool chamber (90), an orifice (70b). Thus, when, due to energization of the actuator, the spool (73) is moved in the spool chamber (90), the open degree of the six orifices (70b) is varied and thus, a hydraulic flow therethrough is controlled. With this operation, a controlled torque transfer is carried out between the drive housing (30) and the rotor (40).
However, due to an inherent construction, the coupling has the following drawback.
That is, as is shown in FIG. 23, the orifice (70b) of each balance fluid passage (70) is circular in shape. This means that the change in the open degree of the orifice (70b) relative to the moved distance of the spool (73) is very small at initial and final stages of the closing or opening stroke of the spool (73) and very large at a middle stage of the stroke. Accordingly, as is seen from the graph of FIG. 24, the raising of the torque transferred during closing stroke of the spool (73) is slow and a very sharp change in the transferred torque "T" appears at the final (or initial) stage of the closing (or opening) stroke of the spool (73). That is, the sharp change in the transferred torque "T" is induced by only a small moved distance of the spool (73) at such final or initial stage of the stroke. As is known to those skilled in the art, this phenomenon brings about a difficulty in achieving a precisely controlled torque transfer particularly when the external actuator is of a common type, that is, a type which fails to give a precise movement to the spool (73) at such final or initial stage of the stroke.
If elimination of such drawback is intended by replacing the common type actuator with a precision actuator, it is inevitably necessary to sacrifice the cost reduction.